Measurement of gas gain fluctuations
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Measurement of gas gain fluctuations. M. Chefdeville , LAPP, Annecy TPC Jamboree, Orsay, 12/05/2009. Overview. Introduction Motivations, questions and tools Measurements Energy resolution & electron collection efficiency Micromegas- like mesh readout

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Measurement of gas gain fluctuations

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Measurement of gas gain fluctuations

Measurement of gas gain fluctuations

M. Chefdeville, LAPP, Annecy

TPC Jamboree, Orsay, 12/05/2009


Overview

Overview

  • Introduction

    • Motivations, questions and tools

  • Measurements

    • Energyresolution & electron collection efficiency

      Micromegas-likemeshreadout

    • Single electrondetectionefficiency and gas gain

      TimePixreadout

  • Conclusion


Gas gain fluctuations

Gas gain fluctuations

  • Final avalanche size obeys a probability distribution

    Signal fluctuations impact on detector performance

    • Spatial resolution in a TPC

    • Energyresolution in amplification-basedgas detectors

    • Minimum gain and ion backflow

    • Detection of single electronswith a pixel chip

  • Whatis the shape of the distribution?

    How doesitvarywithgas, field, geometry…?

  • The Polya distribution parametrized by

    gas gain G and parameterm

    • Works wellwith Micromegas/PPC/MCP/single GEM

    • With GEM stacks, distribution is more exponential

  • Micromegas, NIMA 461 (2001) 84

    = b, relative gain variance

    Micromegas

    T. Zerguerras et al.

    to be published in NIMA


    Gain fluctuations in gas detectors

    Gain fluctuations in gas detectors

    GEM, Bellazzini, IEEE 06, SanDiego

    1 mm gap PPC

    NIMA 433 (1999) 513

    3 GEMs, NIMA 443 (2000) 164

    PPC, Z. Phys. 151 (1958) 563

    Micromegas

    MCP+Micromegas, NIMA 535 (2004) 334


    Investigation methods

    Investigation methods

    • Simulation, since recently within GARFIELD:

      • Simulation of e- avalanche according to

        MAGBOLTZ cross-section database:

        study of gas & field

      • Simulation of e- tracking at microscopic scale

        in field maps (3D):

        study of geometry

    • On the experimental side:

      • Direct measurement of the distribution:

        • High gains, low noise electronics, single electron source

      • Indirect measurements

        • Do not provide the shape but some moments (variance)

        • Assuming Polya-like fluctuations, one obtains the shape

    • In this talk, only indirect methods are presented

      The Polya parameter m is deduced from:

      • Trend of energy resolution and collection efficiency

      • Trend of single electron detection efficiency and gas gain


    Measurement of gain variance

    Measurement of gain variance

    • Energy resolution R and electron collection efficiency η

      • R decreases with the efficiency according to

      • R2 = F/N + b/ηN + (1-η)/ηN

      • R2 = p0 + p1/η

        p0 = (F-1)/N

        p1 = (b+1)/N

      • Measure R(η) at e.g. 5.9 keV, fix F and N, adjust b (i.e. m) on data

    • Single electron detection efficiency κ and gas gain G

      • κ increases with G as more avalanches end up above the detection threshold t

        Integral can be calculated for integer value of m

      • Count the number of e- from 55Fe conversions (N) with TimePix

      • Measure N(G), adjust κ(G,m) on this trend, keep m for which the fit is best


    Experimental set up s

    Experimental set-up(s)

    • Measure 1: R(η)

      • InGrid on bare wafer

      • Preamp/shaper/ADC

      • 55Fe 5.9 keV X-ray source

      • Ar-based gas mixtures

        with iC4H10 and CO2

    • Measure 2: κ(G)

      • InGrid on TimePix chip

      • Pixelman and ROOT

      • 55Fe 5.9 keV X-ray source

      • Enough diffusion for counting

        • 10 cm drift gap

        • Ar 5% iC4H10


    Energy resolution collection

    Energy resolution & collection

    • Vary the collection efficiency with the field ratio

    • Record 55Fe spectra at various field ratios

      • Look at peak position VS field ratio

        define arbitrarily peak maximum as η = 1

      • Look at resolution VS collection

      • Adjust b on data points


    Energy resolution collection1

    Energy resolution & collection

    • Fit function:

    • R = √(p0 + p1/η)

    • p0 = (F-1)/N

    • p1 = (b+1)/N

    • F = 0.2 in all mix.

    • N = 230 in Ar/iso

    • N = 220 in Ar/CO2

    • Record 55Fe spectra at various field ratios

      • Look at peak position VS field ratio

        define arbitrarily peak maximum as η = 1

      • Look at resolution VS collection

      • Fix F and N, adjust b on data points

    • Rather low Polya parameter 0.6-1.4

    • May be due to a poor grid quality

    • Curves do not fit very well points

    • Could let F or/and N free


    Single electron detection efficiency and gain

    Single electron detection efficiency and gain

    (TimePix) threshold

    • Trend depends on:

      • the threshold t, the gas gain G and m


    Single electron detection efficiency and gain1

    55Fe X-ray E0: 5.9 and 6.5 keV

    Photo-e-

    Auger-e-

    TimePix chip

    Single electron detection efficiency and gain

    • Considering55Fe conversions:

      the efficiencyisproportionnal to the number of detectedelectronsat the chip

    • Count the number of electronsatvarious gains

      • In the escape peak!

      • Applycuts on the X and Y r.m.s. of the hits

    55Fe cloud in Ar5iso after 10 cm drift


    Single electron detection efficiency and gain2

    Single electron detection efficiency and gain

    • Number of detected electrons at given voltage determined by

      • Adjusting 2 gaussians on escape peak

      • Kbeta parameters constrained by Kalpha ones

      • 3 free parameters

    • Number of detected electrons and voltage

      • Use common gain parametrization

      • Fix p2 (slope of the gain curve)

      • 2 free parameters: t/A and ηN


    Single electron detection efficiency and gain3

    Single electron detection efficiency and gain

    Best fit for m = 3

    Yields √ b =1/√m ~ 58 %

    m rms (%) Chi2

    1 100 39.43

    2 71 3.08

    3 58 1.16

    4 50 1.41

    5 45 1.60

    • Also, ηN = 115 e-

    • Upper limit on W(Ar5iso) < 25 eV

    • Correcting for un-efficiency:

    • Upper limit on F(Ar5iso) < 0.3


    Conclusion

    Conclusion

    • Two methods to investigate gas gain variance

      • Assuming Polya fluctuation, shape available for detector simulation

      • Energy resolution and collection efficiency

        simple (mesh readout)

        but a certain number of primary e- and Fano factor have to be assumed

      • Single e- detection efficiency and gain

        powerful (provide not only m but W and F)

        but a InGrid-equipped pixel chip is needed

    • Another one not presented

      • Energy resolution and number

        of primary electrons

    4 main lines in an 55Fe spectrum

    • R2 = p0/N

    • p0 = F+b


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